Detection limit improvement for iodine Cordes-Band based hypersonic density measurements

Measurements in the wake region created by models in supersonic and hypersonic flows are required in order to understand a variety of problems in aerodynamics. In this experiment we investigated the properties of using broadband excitation, as opposed to narrowband excitation, using an ArF laser as a means for increasing the detection limit of density measurements in air. The mechanism for this involves Iodine Cordes-Band spectroscopy. Laser excitation of the Cordes bands of I$_{2}$ and the resulting emission involve a myriad of rotational, vibrational and electronic energy levels. I$_{2}$ is optically pumped to the D state where approximately 85{\%} of the emission regardless of buffer gas pressure results from D and D' transitions. In pure I$_{2}$ McLennan-band emission dominates with the peak signal near 321nm. As the air pressure increases, collisions with air transfer the population from the D to D' state where that emission produces the 340nm band. As the air pressure increases the 321nm emission is rapidly quenched and the D' emission dominates (340nm). This provides a pressure dependent signal contribution. The goal is to improve the current detection limit by an order of magnitude by using a broadband laser beam instead of the narrowband beam. This should increase the detection limit by an order of magnitude.